The purpose of this study is to investigate and clarify the relationship between physical illness and depression among the elderly. Although there exists strong support for the conclusion that depressive disorders are closely related to physical illness, none of the existing studies answer important questions about the mediating mechanisms which may account for this relationship. Two general types of explanations have been suggested to explain the co-ocurrence of depression and physical illness: the biological factors explanation argues that symptoms of depression are a direct consequence of illness or a side effect of medication; the psychosocial stress factors explanation emphasizes the occurrence and personal significance of the stress associated with being physically ill. To test the relative importance of these factors, a prospective study is proposed in which 300 geriatric outpatients will be assessed at six month intervals over a period of eighteen months. All patients will undergo detailed medical evaluations and complete a psychosocial survey designed to assess individual and disease characteristics that may play a role in the relationship between physical illness and depression. Multivariate analytic techniques and structural modeling procedures will be used to analyze the longitudinal data and to test causal models linking physical illness with depression.